Catalyst combinations used for the polymerization of alpha-olefins, particularly propylene, are judged by at least two criteria: their activity and yield on catalyst and their ability to produce highly crystalline polymer product with a minimum of coproduced low molecular weight and, particularly, amorphous polymer. While a number of various factors have been found to play a role in increasing activity and yield and in decreasing solubles, one such factor in catalyst combinations using organoaluminum compounds and titanium trichloride is the crystalline form of the titanium trichloride.
There are apparently four crystalline modifications of titanium trichloride, alpha, beta, gamma, and delta, and the powder diffraction patterns for at least the first three have been given in detail in the literature. The purple crystalline modifications, in particular the gamma form, have been preferred for commercial propylene polymerization, while the brown beta form has been consistently rejected in view of its activity and, particularly, its large production of solubles. However, in U.S. Pat. No. 3,424,774 its use has been claimed for the cyclotrimerization of conjugated diolefinic materials.
Recently, a porous, high surface area titanium trichloride based upon violet, delta titanium trichloride has been disclosed in South African Pat. No. 721,245. In combination with organoaluminum compounds, the catalyst is extremely active and produces high yields of propylene polymer without an accompanying large increase in solubles.
Also, U.S. Pat. No. 3,769,233 describes a method for obtaining a "catalyst consisting of violet TiCl.sub.3 . . . from brown TiCl.sub.3 produced by the reduction of TiCl.sub.4 at a temperature below about 100.degree. C., by contacting brown TiCl.sub.3 with TiCl.sub.4." The objective is to convert a known, low surface area, brown form of TiCl.sub.3 into one of the more polymerization-effective violet forms, which violet forms are generally known for their higher polymerization rates and yields and higher stereospecificities.
In U.S. Pat. No. 3,058,963 polymerization using a "hydrocarbon-insoluble reaction product catalyst component" produced by treatment, for example, of TiCl.sub.4 with an alkylaluminum compound in solution is taught.
In U.S. Pat. No. 3,116,274 ethers to promote the stereospecificity of olefin polymerization are used. The preparative process used therein involves mixing an ether, titanium or vanadium tetrachloride and an aluminum alkyl, e.g. aluminum triethyl or sesquichloride, together and combining the reaction product thereof with a promoter for use in alphaolefin polymerization.
Finally, U.S. Pat. No. 3,058,970 teaches improvement of the stereospecificity of olefin polymerization using a catalyst component made by reacting, e.g., titanium tetrachloride and diethylaluminum chloride in the -20.degree. C. to 40.degree. C. temperature range and subsequently aging the result at 40.degree. C. to 150.degree. C.
Now it has been found possible to prepare a highly active, high yield, low solubles producing, brown form of titanium trichloride, despite many literature indications to the contrary, which is very useful as a catalyst component in alpha-olefin polymerization.